Global warming may boost world temperatures by up to 11 degrees
Fahrenheit by the end of the 21st century, a figure substantially higher
than previous estimates, according to a confidential draft report
prepared by an influential group of climate scientists sponsored by the
United Nations.

Moreover, "there is now stronger evidence for human influence on
global climate," the scientists concluded in their preliminary report,
which was distributed to more than 100 governments this week for review.

Several scientists familiar with the new report, prepared by an
international group known as the Intergovernment Panel on Climate Change,
said its findings significantly strengthen the case for a human role in
climate change. Although there is general agreement that the climate is
warming, the question of how much of the change is caused by human action
has been a major topic of scientific inquiry.

The issue has also figured in the presidential campaign. Vice
President Al Gore has frequently asserted that global warming is a major
problem on which the government must begin taking action.

Texas Gov. George W. Bush has been more skeptical. "I don't think
we've got all the facts," he said in the second campaign debate earlier
this month. "I think it's an issue that we need to take very seriously,
but I don't think we know the solution to global warming yet."

In the new draft report, the scientists conclude that it is "likely"
that human actions "have contributed substantially" to the observed
warming. The major human contribution is the release of so-called
greenhouse gases into the atmosphere through the burning of coal, oil,
natural gas and other fossil fuels.

That "is a stronger conclusion" than was offered by earlier
assessments, said Kevin Trenberth, head of the climate analysis section
at the National Center for Atmospheric Research in Boulder, Colo. "This
is not the work of one individual scientist. This is a consensus reached
across scientists in the international community. It has gone through
extensive reviews."

The report is not likely to quiet all debate on the issue, however.
Some longtime critics of projections about global warming said that they
remain skeptical.

Paradoxically, the report also suggests that some pollution control
efforts may unintentionally be making the planet a hotter place.

Greenhouse gases can contribute to global warming by trapping solar
heat and preventing it from being reflected back into space.

Mounting evidence suggests that the Earth's atmosphere has been
steadily warming for nearly 150 years as a result of carbon gas produced
by burning oil, gas and coal, with the warmest years on record occurring
in the last decade.

An 11-degree shift in average temperatures would be a major change in
climate. This week, for example, 11 degrees was the difference in average
temperatures between Los Angeles and Seattle.

The greater increase is projected in large measure because of efforts
to control pollution from industrial facilities and power plants.
Pollution-control measures have greatly reduced the amount of sulfate
particles that cause acid rain and a variety of health problems. But
those particles also have a cooling effect in the atmosphere because they
deflect the sun's heat. As sulfate levels drop, the temperature will
effectively rebound.

"These sulfate particulates have had a masking effect," said one
atmosphere expert who has seen the report but who asked not to be
identified. "We are cleaning up this air pollution, and that is making
global warming worse."

The new report, which will not be made final until it has been
approved early next year, is the first formal update in five years of an
assessment prepared by the climate change panel. The panel is a technical
group sponsored by the U.N. and the World Meteorological Organization and
comprising hundreds of scientists who assess scientific, social and
economic aspects of global climate change. The panel does no original
research of its own but attempts to arrive at a measured technical
assessment of often-conflicting studies on climate change.

The contents of the report were first disclosed Wednesday by the
Associated Press. Several scientists familiar with it agreed to discuss
the findings with The Times, on condition they not be identified.

With international negotiations now underway to limit the amount of
such greenhouse gases that nations may release, almost any attempt to
reach a scientific consensus on climate change is controversial. So far,
no major industrial nation has ratified an agreement negotiated in Kyoto,
Japan, in 1997 to stave off global warming by reducing greenhouse
emissions.

The U.N. group's first report on climate change, released in 1995,
generated considerable criticism over allegations that political bias had
colored its assessments.

Scientists who worked on the report were at pains to rebut such
charges in advance this time. "This is a cold-eyed, objective rendition
of the science," said one such climate scientist. "We could not do it any
better."

To ensure that the new draft is based fairly on the scientific data,
"there have been skeptics involved, as authors and reviewers," Trenberth
said.

The report will be the subject of an international meeting next year
in China, where the dozens of participating governments will all have the
opportunity to review and perhaps temper its conclusions.

"Until that point, the whole report is not considered final,"
Trenberth said. The final report, he said, "involves a negotiation
between the scientists who determine what can be said and the governments
who determine how it can be said."